It's because depending on what you wear, it can be difficult to carry certain guns, or so people say. It's the idea that you should carry the biggest gun you can comfortably carry. In some warmer places, where your attire is likely T-Shirt and Shorts, it's hard to conceal anything larger than an LCP or something similar. Likewise, in colder places, where a heavy jacket is normal wear (or at least is part of the year) it's easy to conceal a full sized service pistol.

I live in Southern Arizona, and I have no trouble concealing my Glock 19 in any weather. With the right holster, and as long as you're dressing around the gun (and not the other way around, which a lot of people who claim they can't carry a larger pistol in warm weather, do) most people should have no difficulty carrying larger guns. Even when it's 110 degrees outside, I have zero difficulty concealing a gun.